So I thought I'd give it a few days before updating so as not to jinx it but the problem appears to be cured. It appears the black lead connecting to the positive terminal may not have been making good contact, the specialists were able to exactly replicate the symptoms by just removing it, the end of the 150A flat fuse below it was looking a bit sorry for itself so they cleaned that up and touch wood it's been fine since. Battery is good and drain after 30mins is 0.01A. I also noticed yesterday the engine fan was working (hadn't realised I'd not heard that recently if that makes sense), I did check the car about an hour later to make sure it had definitely stopped.
They advised the next step should the issue persist is start dismantling the car to trace where that lead goes as after it disappears into the body of the car, they estimated 4-5 hours labour and were reluctant to charge me for that if it doesn't need to be done.
Probably just in my head but it feels nicer to drive and more responsive.
Sorry to hear about you needing a replacement CCM, that's one invoice I'm dreading